Cameras, such as fixed cameras and pan/tilt/zoom (PTZ) cameras, utilize optical systems for focusing a captured image on objects of interest. In a camera optical system, there are different lenses for zoom (referred to herein as a “zoom lens”) and focus (referred to herein as a “focus lens”). The zooming factor is determined by the zoom lens, whereas whether the image is focused is determined via the focus lens. The camera system may utilize a varifocal lens, which is a camera lens with variable focal length in which focus changes as focal length (and magnification) changes, as compared to a parfocal zoom lens, which remains in focus as the lens zooms (i.e., the focal length and magnification change).
Under good lighting conditions, the aperture will be small, and thus the depth of the field will be relatively wide. As a result, the camera may produce a focused image under a wide range of focus lens positions. However, under less optimal lighting conditions, the aperture becomes wider, and thus the depth of field becomes narrower. In this case, focus lens position may have a larger impact on image focus. If the zoom lens is in the high zooming factor region and the depth of field also narrows, correct focus lens position has a significant impact on producing a good focused image. When both bad lighting conditions and a high zooming factor are present, the depth of field becomes narrower and the focus lens position again has a significant impact on image focus. As focus issues are most prevalent in low light conditions, a high zooming factor is used.